Use DI to implement battery detection, add BatteryCutoff property

- Removed the Xamarin.Essentials package from osu.Game and added it to osu.iOS and osu.Android only.
- iOS and Android implementations use Xamarin.Essentials.Battery, while the Desktop implementation
only returns 100% battery for now.
- Added a BatteryCutoff property to PowerStatus so it can be different for each platform (default 20%, 25% on iOS)

osu.Game/Utils/PowerStatus.cs

@@ -0,0 +1,22 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+namespace osu.Game.Utils
+{
+    public abstract class PowerStatus
+    {
+        /// <summary>
+        /// The maximum battery level before a warning notification
+        /// is sent.
+        /// </summary>
+        public virtual double BatteryCutoff { get; } = 0.2;
+        public virtual double ChargeLevel { get; set; }
+        public virtual bool IsCharging { get; set; }
+    }
+
+    public class DefaultPowerStatus : PowerStatus
+    {
+        public override double ChargeLevel { get; set; } = 1;
+        public override bool IsCharging { get; set; } = true;
+    }
+}